The subject of the present invention is a device for holding lateral tabs of box blanks passing through a folder-gluer, the machine commonly used in the packaging industry to make up boxes from cardboard blanks.
A folder-gluer machine generally comprises a series of modules, each intended to carry out a given operation so that the box blanks introduced at the input of the machine in a feeder, reemerge in the form of folded boxes ready for use. Typically, the operations carried out in a folder-gluer are in sequence: pre-breaking of certain fold lines of the blank, folding of parts such as the lateral tabs, folding of the front and rear tabs, coating the tabs with glue, pressing of the blanks once folded and glued, and their receipt at the end of the production line.
The box blanks are generally conveyed through the various modules of the machine via a belt conveyor which, through friction, holds the blanks either between lower and upper belts or between lower belts and upper press rollers.
It is in the module that folds the rear tabs that the device of the present invention finds its use. The folding of the rear tabs is a relatively complicated operation performed by one or more horizontal-axis rotary folders. These folders generally comprise two hooks mounted in opposition on a central hub which are able to move along a drive shaft positioned transversely to the direction of travel of the blanks. In a movement that is synchronized with the rate of travel of these blanks, these hooks each in turn emerge from the plane of passage of the blanks, in the space separating two consecutive blanks. Each hook is temporarily given a speed higher than the speed at which the blanks are moving, so that the hook is able, at a given moment, to catch the rear tab of the blank and fold it completely onto the rear part of the box blank.
To be able to carry out this folding operation, it is generally necessary to interrupt the belts or rollers of the upper conveyor which holds the lateral parts already folded, pressed against the lower conveyor. Without the action of the upper conveyor, these lateral tabs have the tendency to open up. This effect is more pronounced when the rear tab is being folded, during which operation, the rear of the blank is predisposed to being lifted by the rotary folder.
To prevent the lateral tabs from opening out or the lifting of the blank at the time of the folding of the rear tab, recourse to rotary supports is known. Such supports are described in patent CH 457 114 that describes a similar device. These supports consist of two diametrically opposed flexible fins or blades mounted in continuous rotation about a drive shaft so as to form a helix. The rotational speed of this shaft is directly dependent on the rate of travel of the blanks through the machine. To achieve this, the drive shaft is mechanically connected by a drive line either to the drive of the rotary hooks or to the main drive of the folder-gluer or alternatively to a so-called “synchronous” feeder so that it is always in perfect synchronicity with the rate at which the blanks pass through the machine.
A synchronous feeder is a mechanical entity that is able to synchronize the start of the blanks by means of feed guides mounted on toothed belts. This feeder is said to be “synchronous” because of its function that compensates for any delay on the part of the blank as a result of unwanted slippage against the conveyor.
The chief disadvantage with such a device for holding the lateral tabs lies in the fact that its operation is completely dependent on its drive system. This dependency prevents any time delay between the enaction of the rotary supports with respect to the folding of the rear tab by the rotary hook. Now, it has been found that, depending on the type of box being made up, the lateral tabs are not always positioned the same distance away from the rear tab. In such cases, the folded lateral parts of the blank are not always held in place at the most opportune moment.
According to another device known to those skilled in the art, the supports are not mounted on a spindle in continuous rotation but on the leaf of a hinge that allows the support to move back and forth. This support moves from one position to another by means of a return spring for example. The chief disadvantage with such a system lies in the fact that the mechanism is not rapid and reliable enough to sustain high production rates.
Another solution might be to provide a mechanical cam in the drive of the rotary supports, a mechanical cam allowing a predefined delay or advance to be introduced into the action of the rotary supports, depending on the boxes being processed. Such a solution would, however, suffer from a major disadvantage associated with the fact that it would then become necessary to provide as many specific cams as there are types of box to be processed. From an economic standpoint, such a solution is of course inconceivable.